In mass spectrometry, it is desirable to be able to accurately identify the apex of an elution peak for a particular mass-to-charge ratio. As one typical example, tandem mass spectrometry is a technique that utilizes two or more successive stages of mass analysis with a collision or reaction process occurring between each stage. Two-stage tandem mass spectrometry is typically referred to as mass spectrometry/mass spectrometry (MS/MS). In a data dependent mode, the eluting sample can be automatically selected for further analysis in the second stage when the intensity of a mass spectral peak from the first stage is above a user-specified threshold. Although this type of threshold-based approach has been generally adequate for its intended purposes, it has not been satisfactory in all respects.
For example, a user will often set the threshold at a relatively low value, in order to avoid missing any mass spectral peak that might be of interest. However, this results in masses being selected as soon as they elute and appear above the threshold, or in other words in advance of the occurrence of the apex of the chromatographic peak. As a result, analysis often occurs before the apex of the peak. However, the best spectra can be obtained in the shortest amount of time at the apex of the peak, because the analyte is most concentrated at this point. Unfortunately, it is not a simple matter to identify the apex of a chromatographic peak in real time.
For example, once a peak exceeds a user threshold, the peak could be put on a watch list, and a selection of that peak for second-stage analysis could be delayed while the intensity continues to increase. Then, when the intensity begins to decrease, the peak could be selected. Unfortunately, however, chemical noise and source instability can cause fluctuations in the rising edge of the chromatographic peak. In other words, the rising edge may have an irregularity that causes a temporary decrease, even though the analyte concentration is actually still increasing. To reduce this type of problem, it would be possible to apply smoothing techniques to the data obtained from the first-stage analysis. However, real-time smoothing techniques have a tendency to introduce a phase lag, and the phase lag can create a situation where the smoothed data does not begin decreasing in intensity until after the analyte has completed eluted. In other words, by the time the analyte is selected for further analysis, the chromatographic peak has passed and the analyte is no longer present.